Aqueous dispersion material containing solid particles has been known as a functional material, examples of which include an agrochemical such as a herbicide or an insecticide, a pharmaceutical such as an anticancer drug, an anti-allergy drug or an anti-inflammatory drug, or a coloring material such as ink or toner containing a colorant as the solid particles. On the other hand, digital printing technologies have shown a remarkable progress in recent years. The digital printing technologies, represented by an electrophotographic technology and an ink jet technology, are showing increasing importance as the image forming technology for use in offices and homes.
Among such technologies, the ink jet technology has major advantages, as a direct recording method, of compactness and a low electric power consumption. Also, improvement in image quality is progressing rapidly, for example, by a fine nozzle formation. An example of the ink jet technology is a method of heating ink, supplied from an ink tank, with a heater provided in a nozzle, thereby generating a bubble in the ink and discharging the ink to form an image on a recording medium. Another example is a method of causing a vibration in a piezoelectric element for discharging the ink.
Since such ink jet technology generally employs an aqueous solution of a dye as the ink, there may result a diffusion of the image when colors are superposed, or a so-called feathering phenomenon may result along fibers of paper, in a recorded spot on the recording medium. Use of a pigment-dispersion ink is being considered in order to alleviate these phenomena. There are proposed a method of dispersing a pigment by a nonionic polymer, thereby sterically suppressing coagulation of the pigment, and, as disclosed in U.S. Pat. No. 5,085,698, a method of dispersing a pigment by an ionic block polymer including one each of a hydrophilic component and a hydrophobic component. Further improvements are desired, however, in dispersibility, by suppressing the coagulation resulting from an interaction of the particles, thereby achieving stable dispersion in a solvent over a prolonged time; in a color hue; and in a color-developing property.